Sounds like a police turn in. The B, your guess is as good as mine. The DPS could either be Department of Public Safety (that is what they call the highway patrol in Texas and other areas I imagine), though more likely it stands for District Park Service. Many park services issued interesting pistols not usually found in service to LE. Idaho Fish and Game issued the H&K P9S for example. I would suspent the 45 to be the number of the pistol issued. Many departments do this starting with 1 on up to how many they have. The fact that yours is 45 rather than 045 would tell me that there were less than 100 pistols marked like this. This is speculation so don't hold me to it.
